backend: refine min|max mag
backend: refine min|max mag

do it according to spec to make cft pass

Signed-off-by: rander.wang <5299db4a9bf784920a3e558099f6736bcd8939e6@intel.com>
Tested-by: Yang Rong <78759e37dc7114caed18f74e7515c5cb519dbf6d@intel.com>

wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet,wdv4758h/beignet